1. Field of the Invention
The invention relates to a continuous steel casting plant for billet and bloom formats.
2. Description of Related Art
Long continuous casting products are cast predominantly in tubular permanent molds with a rectangular, and often with an approximately square or round, cross-section. The billet and bloom slabs are then further processed by rolling or forging.
For producing continuous casting products with good surface and texture quality, in particular billet and bloom slabs, a uniform heat transition along the circumferential line of the slab cross-section between the slab being formed and the wall of the die cavity is of crucial importance. Many proposals are known for designing the geometry of the die cavity, in particular in the areas of the corner fillets of the die cavity, in such a way that no damaging air gaps arise between the slab shell being formed and the wall of the permanent mold, causing an uneven heat transition along a circumferential line of the slab cross-section and solidification defects and fractures.
Corners of the die cavity of tubular permanent molds are rounded by fillets. The larger the configuration of the fillets in the die cavity of the permanent mold, the more difficult it is to achieve a uniform cooling between a slab shell being formed and the walls of the permanent mold, in particular over the circumference of the die cavity. The incipient solidification of the slab just below the bath level in the permanent mold proceeds differently on straight sections of the circumference of the die cavity from the fillet areas. The heat flow on the straight or substantially straight sections is quasi one-dimensional and follows the law of heat transmission through a flat wall. In contrast to this, the heat flow in the rounded corner areas is two-dimensional and it follows the law of heat transmission through a curved wall.
The resulting slab shell is normally thicker in the corner areas at the start of solidification below the bath level than on the straight surfaces and begins to shrink sooner and more intensely. The result of this is that even after about 2 seconds the slab shell lifts up irregularly from the wall of the permanent mold in the corner areas and air gaps form, which drastically impair the heat transmission. Not only does this impairment of the heat transmission delay the further growth of the shell, but it can even cause a re-fusion of already solidified inner layers of the slab shell. This fluctuating pattern of the heat flow—cooling and re-heating—leads to slab defects such as surface and internal longitudinal cracks at the edges or in areas near the edges, and also to mold defects such as rhomboidity, indents, etc. A re-fusion of the slab shell or larger longitudinal cracks can also lead to fractures.
The larger the fillets are dimensioned compared with the side length of the slab cross-section, in particular if the fillet radii amount to 10% or more of the side length of the die cavity cross-section, the more frequently such slab defects occur. This is one reason why the fillet radii are usually limited to 5 to 8 mm, although larger roundings at the slab edges would be more favorable for the subsequent rolling.
During casting at high casting speeds the dwell time of the cast slab in the permanent die cavity is reduced and the slab shell has overall less time to grow in thickness. Depending on the slab format chosen it is therefore necessary to support the slab shell with support rollers immediately after it leaves the permanent mold in order to avoid bulging of the slab shell or even fractures. Support roller stands of this kind directly beneath the permanent mold are exposed to great wear and can be restored to service after a fracture only with great expenditure of time and cost.
A permanent mold for continuous casting of billet and bloom slabs is known from JP-A-11 151555. In order to avoid rhomboid deformation of the slab cross-section when casting rectangular slabs and in order additionally to increase the casting speed, the fillets are specially shaped at the four corners of the die cavity as so-called corner cooling parts. On the pouring-in side the corner cooling parts are constructed as circular recesses in the wall of the permanent mold, which become smaller in the moving direction of the slab and re-form to a corner fillet towards the exit of the permanent mold. The degree of curvature of the circular recesses increases in the moving direction of the slab towards the exit of the permanent mold. This shaping is intended to ensure uninterrupted contact between the corner area of the slab shell and the specially shaped corner cooling parts of the permanent mold.
From JP-A-09 262641 a tubular permanent mold is known for the continuous casting of rectangular slabs, which in order to avoid longitudinal cracks at the slab edges and rhombus-shaped slab cross-sections in the die cavity, employs fillets with different corner radii at the upper and lower end of the permanent mold. The upper corner radius at the inlet side of the permanent mold is chosen to be smaller than the corner radius at the outlet side of the permanent mold. This measure is said to avoid an air gap between the slab shell and the wall of the permanent mold. No details are given or implied regarding the size of the fillets in relation to the side length of the slab cross-section and the absolute size of the slab cross-section, nor is any information given or implied concerning simplifying the support guidance adjoining the permanent mold.